Recently, there has been developed a roller screw having rollers movably interposed between a screw shaft and a nut. As each roller is in line contact with a roller rolling surface, the roller screw has the advantage of larger load to bear as compared with a ball screw. However, although a ball can roll in every direction, the roller can move only in one direction and therefore, there is a problem that the roller is very difficult to circulate.
The roller screw has a circulation path having a spiral loaded roller rolling path between a spiral roller rolling surface formed on an outer peripheral surface of the screw shaft and a spiral loaded roller rolling surface formed on an inner peripheral surface of the nut and an unloaded return path connecting one end of the loaded roller rolling path to the other end. The unloaded return path is formed in a circulation member which is mounted on the nut. In the circulation member, a lip portion for scooping rollers moving on the roller rolling surface of the screw shaft is provided as a scooping portion for scooping the roller rolling on the spiral loaded roller rolling path. The lip portion juts from the nut toward the roller rolling surface of the screw shaft.
In the unloaded return path, clearance is provided around the rollers and the diameter of the unloaded return path is larger than the diameter of the loaded roller rolling path. In a conventional roller screw, in order to make up for a difference between diameters of the unloaded return path and the loaded roller rolling path, the inner wall surface of the lip portion of the circulation member is formed tapered with the width that gradually becomes narrower from the unloaded return path toward the loaded roller rolling path (see patent document 1). Then, the rollers moving in the unloaded return path are first aligned to be guided to the loaded roller rolling path.
As described above, the rollers moving in the spiral loaded roller rolling path are scooped by the scooping portion of the circulation member and guided into the unloaded return path. In the conventional roller screw, in order to smoothly scoop each roller moving on the loaded roller rolling path, the unloaded return path where the scooping portion is formed is arranged in the tangential direction of the loaded roller rolling path (see patent document 2). That is, it is arranged in the tangential direction of the circular loaded roller rolling path seen in the axial direction of the nut and in the lead angle direction of the loaded roller rolling path seen in the side surface direction of the nut.    [Patent document 1] Japanese Patent Application Laid-Open No. 2006-118649    [Patent document 2] Japanese Patent Application Laid-Open No. 11-210858